1. Field of the Invention
The invention is related to the field of magnetoresistance (MR) elements and, in particular, to dual-type tunneling magnetoresistance (TMR) elements and associated methods of fabrication, where the pinned layer structures of the TMR elements include different materials so that the Fermi levels of the pinned layer structures are different.
2. Statement of the Problem
Many computer systems use magnetic disk drives for mass storage of information. Magnetic disk drives typically include one or more recording heads (sometimes referred to as sliders) that include read elements and write elements. A suspension arm holds the recording head above a magnetic disk. When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) side of the recording head to ride a particular height above the magnetic disk. The height depends on the shape of the ABS. As the recording head rides on the air bearing, an actuator moves an actuator arm that is connected to the suspension arm to position the read element and the write element over selected tracks of the magnetic disk.
To read data from the magnetic disk, transitions on a track of the magnetic disk create magnetic fields. As the read element passes over the transitions, the magnetic fields of the transitions modulate the resistance of the read element. The change in resistance of the read element is detected by passing a sense current through the read element, and then measuring the change in bias voltage across the read element. The resulting read back signal is used to recover the data encoded on the track of the magnetic disk.
The most common type of read elements are magnetoresistance (MR) read elements. One type of MR read element is a Giant MR (GMR) read element. GMR read elements using two layers of ferromagnetic material (e.g., CoFe) separated by a nonmagnetic spacer layer (e.g., Cu) are generally referred to as spin valve (SV) read elements. A simple-pinned SV read element generally includes an antiferromagnetic (AFM) pinning layer (e.g., PtMn), a ferromagnetic pinned layer (e.g., CoFe), a nonmagnetic spacer layer (e.g., Cu), and a ferromagnetic free layer (e.g., CoFe). The ferromagnetic pinned layer has its magnetization fixed by exchange coupling with the AFM pinning layer. The AFM pinning layer generally fixes the magnetic moment of the ferromagnetic pinned layer perpendicular to the ABS of the recording head. The magnetization of the ferromagnetic free layer is not fixed and is free to rotate in response to an external magnetic field from the magnetic disk.
Another type of SV read element is an antiparallel pinned (AP) SV read element. The AP-pinned SV read element differs from the simple pinned SV read element in that an AP-pinned structure has multiple thin film layers forming the pinned layer structure instead of a single pinned layer. The pinned layer structure includes a first ferromagnetic pinned layer (e.g., CoFe), a nonmagnetic spacer layer (e.g., Ru), and a second ferromagnetic pinned layer (e.g., CoFe). The first ferromagnetic pinned layer has a magnetization oriented in a first direction perpendicular to the ABS by exchange coupling with the AFM pinning layer. The second ferromagnetic pinned layer is antiparallel exchange coupled with the first ferromagnetic pinned layer across the spacer layer. Accordingly, the magnetization of the second ferromagnetic pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetization of the first ferromagnetic pinned layer.
Another type of MR read element is a Tunneling MR (TMR) read element. TMR read elements differ from GMR elements in that a thin, electrically insulating, tunnel barrier layer (e.g., aluminum oxide or magnesium oxide) is used between the ferromagnetic pinned layer and the ferromagnetic free layer instead of a nonmagnetic spacer layer (e.g., Cu). The TMR read elements may be simple pinned or AP-pinned as with the GMR read elements.
GMR read elements and TMR read elements may be current in plane (CIP) read elements or current perpendicular to the planes (CPP) read elements. Read elements have first and second leads for conducting a sense current through the read element. If the sense current is applied parallel to the major planes of the layers of the read element, then the read element is termed a CIP read element. If the sense current is applied perpendicular to the major planes of the layers of the read element, then the read element is termed a CPP read element.
The above GMR and TMR read elements may be referred to as single-type GMR read elements and single-type TMR read elements. To improve the performance of the read elements, some recording head manufacturers are developing dual-type GMR read elements and dual-type TMR read elements. A dual-type TMR read element, for example, includes two tunnel barrier layers on either side of the ferromagnetic free layer. The dual-type TMR read element also includes two ferromagnetic pinned layers, with one ferromagnetic pinned layer adjacent to each of the tunnel barrier layers. Dual-type TMR read elements and dual-type GMR read elements have the effect of increasing the rate of resistance change in the element as compared to single-type elements, which allows the dual-type elements to be more sensitive to external magnetic fields.
FIG. 1 illustrates a typical dual-type TMR read element 100. FIG. 1 is a view from the-air-bearing surface (ABS) of TMR read element 100. TMR read element 100 includes a TMR read sensor 110 sandwiched between a first shield 131 and a second shield 132. TMR read sensor 110 is a TMR read sensor used in a current-perpendicular-to-plane (CPP) mode.
TMR read sensor 110 includes a nonmagnetic seed layer 111 (e.g., Ta), an antiferromagnetic (AFM) lower pinning layer 112 (e.g., PtMn), a ferromagnetic CoFe lower pinned (keeper) layer 113, a nonmagnetic Ru spacer layer 114, a ferromagnetic CoFe lower pinned (reference) layer 115, a tunnel barrier layer 116, and a ferromagnetic CoFe free layer 117. TMR read sensor 110 also includes a tunnel barrier layer 118, a ferromagnetic CoFe upper pinned (reference) layer 119, a nonmagnetic Ru spacer layer 120, a ferromagnetic CoFe upper pinned (keeper) layer 121, an AFM upper pinning layer 122, and a nonmagnetic Ta cap layer 123. Traditionally, the lower portion of TMR read sensor 110 and the upper portion of TMR read sensor 110 (below and above free layer 117) are mirror images of each other in terms of materials used, layer or film thicknesses, etc.
When in operation, a sense current is passed through TMR read sensor 110 in a CPP fashion. The sense current and the resistance of TMR read sensor 110 generates a bias voltage. The change in bias voltage is used to generate a read back signal that shows the data stored on a magnetic disk. One characteristic of TMR read element 100 of FIG. 1 is that the TMR of TMR read element 100 and bias voltage are related such that TMR decreases as the bias voltage increases. This characteristic is due to the band structure of the materials used to form TMR read sensor 110.
FIG. 2 is a graph illustrating TMR versus bias voltage. The graph in FIG. 2 illustrates the TMR response of TMR read sensor 110. If a negative bias voltage is induced in TMR read element 100, then TMR drops as the negative bias voltage increases. Similarly, if a positive bias voltage is induced in TMR read element 100, then TMR drops as the positive bias voltage increases. For instance, at positive bias voltage V1, the TMR of TMR read sensor 110 is still relatively high but voltage V1 may be lower than desired. At positive bias voltage V2, the voltage V2 may be a desirable voltage, but TMR of TMR read sensor 110 has drop significantly. Unfortunately, the characteristic of decreasing TMR with increasing bias voltage is a limiting factor for performance and reliability of TMR read elements.